The present invention relates to a gas compressor and, particularly, to a gas compressor to be used in a Stirling cycle machine which is a thermodynamic cycle machine by which high temperature and low temperature or power is generated externally through repeating Stirling cycle of a working gas filled therein.
An example of application of a conventional gas compressor to a Stirling cycle gas refrigerator will be described.
FIG. 8 is a cross sectional side view of a conventional Stirling cycle gas refrigerator similar to that disclosed in, for example, Japanese Patent Publication No. 28980/1979 or U.S. Pat. No. 3,991,585. In this figure, a compressor portion 31 comprises mainly a cylinder 1 and a piston 2 adapted to reciprocate in the cylinder 1. A cold finger 3 encloses a displacer 4 which is reciprocated by pressure variation of working gas and has a lower portion communicated through a communication tube 5 with the cylinder 1.
In the cold finger, a first compression space 7 is formed between a lower working surface 4a of the displacer 3 and a communication tube 5 and, in the compressor portion, a second compression space 8 is formed between an upper working surface 2a of the piston 2 and the communication tube 5. An expansion space 6 is provided above an upper working surface 4b of the displacer 3. The working surface 4b forms a border of the expansion space 6 which forms, together with the first compression space 7, the second compression space 8 and spaces within a regenerator 9 and the communication tube 5, etc., a working space. The regenerator 9 can be communicated through a center hole 10 of the displacer 3 with lower working gas and through a center hole 11 and a radial flow duct 12 of the displacer 3 with upper working gas. Further, in this refrigerator, a freezer 13 is provided as a heat exchanger for exchanging heat between expanded cold working gas and members to be cooled thereby.
A clearance seal 14 is arranged between the piston 2 and a wall of the cylinder 1 to prevent working gas from flowing between a first buffer space 15 provided on the side of a lower working surface 2b of the piston 2 and the working space. A clearance seal 16 is further provided between the displacer 4 and the cold finger 3 to force working gas between the expansion space 6 and the first compression space 7 to flow through the regenerator 9.
The piston 2 is equipped, on a lower end portion thereof in the first buffer space 15, with a light weight sleeve 17 of non-magnetic material such as aluminum. A coil 18 is wound on the sleeve 17. Opposite ends of the coil 18 are connected through lead wires 19 and 20 passing through the wall of the cylinder 1 to electric terminals 21 and 22 outside the cylinder 1, respectively. The coil 18 can reciprocate in an axial direction of the piston 2 within an annular gap 23 in which an armature magnetic field exists. Magnetic line of force of this armature magnetic field generated by an annular permanent magnet 24 extends radially of a moving direction of the coil 18 from an annular armature 25, through a cylinder 26 to an armature disc 27, which constitute a closed magnetic circuit. The sleeve 17, the coil 18, the lead wires 19 and 20, the annular gap 23, the annular permanent magnet 24, the annular armature 25, the cylinder 26 and the armature disc 27 constitute a linear motor 28 for driving the piston.
Further, the piston 2 and the displacer 4 are resiliently supported in the cylinder 1 and the cold finger 3 through a spring member 29 and a spring member 30, respectively, to fix positions of the piston 2 and the displacer 4 in stationary states thereof and neutral positions during operation.
As mentioned, the cylinder 1, the piston 2, the linear motor 28 and the spring member 29 constitute the gas compressor 31 for producing a pressure variation in the second compression space necessary to generate a cold state and a hot state.
In operation, when an a.c. power source (not shown) having a frequency equal to a resonance frequency of the system is connected across the electric terminals 21 and 22, an a.c. current flows through the coil 18 and the latter is subjected to an axial periodic Lorentz force due to an interaction of the a.c. current and a radial magnetic field produced by the annular permanent magnet 24. As a result, a system composed of the piston 2, the sleeve 17, the coil 18 and the spring member 29 is brought into resonance state and vibrates axially. Vibration of the piston 2 causes a periodic variation of pressure in working gas filled in the working space composed of the expansion space 6, the first compression space 7, the second compression space 8, the communication tube 5, the regenerator 9, the center hole 10, the center hole 11, the radial flow duct 12 and the freezer 13 and causes an axial, periodic and alternative vibration force to be produced in the displacer 4 due to flow rate variation of gas passing through the regenerator 9. In this manner, the displacer 4 including the regenerator 9 reciprocates axially in the cold finger 3 at the same frequency as and with a different phase from that of the piston 2.
When the piston 2 and the displacer 4 operate with a suitable phase difference therebetween, the working gas filling the working space performs a thermodynamic cycle known as "Reverse Stirling Cycle" and generates hot and cold states in the compression space 7 and 8 and the expansion space 6 and the freezer 13, respectively. The "Reverse Stirling Cycle" and the principle of hot and cold state generation are disclosed in detail in "Cryocoolers", G. Walker, Plenum Press, New York, 1983, pp, 177-123. The principle will be described in brief below.
After compression heat of gas in the second compression space 8 generated by compression operation of the piston 2 is dissipated during its passage through the communication tube 5, the working gas flows into the first compression space 7 and passes through the center hole 10 and the regenerator 9 where it is preliminarily cooled in the regenerator 9 by cold state established during a half cycle before. Then the working gas enters through the center hole 11, the radial flow duct 12 and the freezer 13 into the expansion space 6. Once almost all working gas enters into the expansion space 6, expansion commences, resulting in cold state in the expansion space 6. The working gas, then, returns in the reverse direction, while discharging the cold state to the regenerator 9, to the second compression space 8. In this time, the working gas absorbs external heat in the freezer 13 to cool an associated external matter. Then, when almost all working gas returns to the second compression space 8, the compression is restarted to execute a next cycle. The "Reverse Stirling Cycle" is completed according to the process mentioned above to produce cold and hot states.
As will be clear from the foregoings, in the conventional cooler constituted in this way, a cooling performance is generally controlled by changing current to be supplied to the coil 18. That is, the performance is controlled by changing an amplitude of pressure variation within the working space by increasing and decreasing the stroke of the piston 2 by means of a current flowing through the coil 18.
Since the conventional compressor is constituted as mentioned above, in which the neutral point of the piston reciprocation is fixed by the neutral point of the piston spring member, there may be a case where the piston collides with the cylinder when the stroke is varied substantially for the capability control.
Alternatively, in a case where a clearance corresponding to the maximum stroke is given to enlarge the control range, a dead space, i.e., a portion of the compression space in which the piston does not reciprocate, is increased and, therefore, the compression ratio is reduced, resulting in lowered pressure variation per stroke. Therefore, there is a problem that the efficiency of the refrigerator is lowered.